objective-c: determining if a regex matches a string

Question

What's wrong with the following code?

-(IBAction)numFieldDoneEditing{
        NSPredicate * regexTest = [NSPredicate predicateWithFormat: @"SELF MATCHES '^\\d*\\.\\d{2}$'"];
        NSString *text = [[NSString alloc] initWithFormat: @"%@", numberField.text];
        if ([regexTest evaluateWithObject: text] == YES) {
                //do something
        }
        else{
                //do something else
        }
        [text release];
        [regexTest release];
    }

Adam Rosenfield · Accepted Answer · 2009-02-04 22:12:54Z

3

For some reason, you have to escape your backslashes twice for the \d sequence in the regex specification:

NSPredicate * regexTest = [NSPredicate predicateWithFormat: @"SELF MATCHES '^\\\\d*\\.\\\\d{2}$'"];

Some other problems with your code:

You shouldn't be releasing the regexText object because you're not an owner. It wasn't created with a method named init or copy. See the Memory Management Programming Guide for Cocoa
This is more of a style issue, but if you have a boolean variable, don't compare it for equality with YES or NO, it just makes the code harder to understand. Just test it or its inverse directly. For example:
```
// Positive test:
if([regexTest evaluateWithObject: text])
    ; // do stuff

// Negative test
if(![regexTest evaluateWithObject: text])
    ; // do stuff
```

edited Feb 4, 2009 at 22:12

answered Feb 4, 2009 at 21:38

Adam Rosenfield

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3 Comments

Chuck Over a year ago

About BOOLs: It's more than a style thing. There's no guarantee that a BOOL will only be set to YES or NO. Since BOOL is the same thing as a char, it's entirely possible to say BOOL flag = 42. It's bad style, but it does happen, so your code shouldn't ever test against YES.

Tim Frey Over a year ago

Can we replace "for some reason..." with an actual reason?

Nas Banov Over a year ago

@OutlawProgrammer well, it is absurd but: first the C compiler chews up every other \, so @"SELF MATCHES '^\\\\d*\\.\\\\d{2}$'" becomes NSString literal SELF MATCHES '^\\d*\.\\d{2}$'. Then NSPredicate parser has a go at the inner literal and it becomes the regular expression ^\d*.\d{2}$. Which makes me think there is error in @Adam Rosenfield's answer - see the decimal point is un-escaped. It should have been @"SELF MATCHES '^\\\\d*\\\\.\\\\d{2}$'" instead. It's PITA, that's what this is.

Martin Gordon · Accepted Answer · 2009-02-05 18:34:49Z

3

Since you tagged this with iphone and cocoa-touch, it should be worth noting that NSPredicate is not available on the iPhone. It will still work in the simulator, since that uses the desktop frameworks, but it won't work on the iPhone itself.

answered Feb 5, 2009 at 18:34

Martin Gordon

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